The present invention concerns a thermometer or temperature alarm device making use of fibre optics.
It is necessary in numerous supervision tasks within industry and technology to measure temperature in such a way that an alarm is actuated when the temperature has transgressed a given threshold value. Of course such threshold alarm can be obtained from a conventional electronic thermometer by providing electronic circuits for threshold detection so that an alarm is actuated if the preset temperature is surpassed.
There are however various situations in which one would prefer to forgo the use of a so-called electronic thermometer because such instruments as a rule contain an electrically conductive measuring pick-up, or sensor, to which a voltage has been applied. Typical objects of measurement where this consideration is relevant are, for instance, warehouses containing readily inflammable chemicals, e.g. oil tanks and cisterns, powder stores, etc. In such conditions the electrically conductive pick-up leads may give rise to sparking, and they may for instance carry lightning into the storage space, with disastrous results.
It is possible, of course, in such situations to use so-called temperature pick-ups utilizing fibre optics, which continuously follow the development of temperature and which can be programmed to actuate an alarm if a given critical temperature is surpassed. As is commonly known, measuring instruments, such as thermometers, using fibre optics have the inherent advantage that the optic fibre itself is an insulator and thus cannot cause any harmful electrical phenomena, such as short circuits or sparking, which might cause dangerous situations in an environment susceptible to explosion.
However, fibre-optic thermometers of present art have turned out to be rather complicated and expensive, and this has limited their use. On the other hand, thermometers employing conventional galvanic technology, such as thermocouples, thermistors or NTC and PTC resistance elements, offer a rather inexpensive and reliable solution to surveillance problems as long as there is no apprehension regarding risk of fire or explosion. It is possible especially with PTC and NTC resistors to implement a temperature detection which is specifically of the alarm type because the resistance of these resistors changes most powerfully as a function of temperature in various temperature ranges.
The fibre-optic thermometers known at present are based on the principle that light is carried with an optic fibre to a measuring cell and the material in this cell reacts in one way or another to the temperature so that the change of temperature can be observed by optical means.
One way known in the art is to make use of the temperature dependence of the fluorescence phenomenon caused by light. It is a well-known fact that the intensity becomes less with increasing temperature. Some of the commercially available measuring instruments are based on this phenomenon.
It is also known that the properties of so-called liquid crystals are dependent on temperature in such a way that it is possible to construct thermometers using liquid crystals. Liquid crystals cause a kind of interference phenomena in thin films so that the colour of the liquid crystal film appears to change as a function of temperature in desired manner. A type of paint composed of liquid crystals is known which can be used to coat those objects which are of interest, and one may then observe the changing colour by those means which fibre optics afford. It has to be noted, however, that liquid crystal materials have no particularly long life span: in a way, they are decomposed as they age.